The advantages of multiple-input multiple-output (MIMO) systems have been widely acknowledged, to the extent that certain transmit diversity methods have been incorporated into wireless standards. Although transmit diversity is clearly advantageous on a cellular base station, it may not be practical for other scenarios. Specifically, due to size, cost, or hardware limitations, a wireless agent may not be able to support multiple transmit antennas. Recently, a new class of methods called cooperative communication has been proposed that enables single antenna mobiles in a multi-user environment to share their antennas and generate a virtual multiple-antenna transmitter that allows them to achieve transmit diversity.
FIG. 1 shows a preliminary explanation of cooperative communication. The basic communication elements for cooperative communication systems are two mobile agents and one base-station communicate via independent fading channels. Although each mobile has one antenna and cannot individually generate special diversity, cooperative techniques make it possible for one mobile to forward some version of information for the other, thus achieving spatial diversity.
FIG. 2 shows general representation of cooperative communication, which includes source node (S), relay node (R) and destination node (D). The three channels between them are called direct channel 202, interlink channel 204 and relay channel 206 correspondingly. Various schemes have been proposed to explore the benefits of cooperative communications. Existing two categories of cooperative communication are amplify-and-forward (AF) and decode-and-forward (DF). The baseband discrete time signals received at destination node and relay node can be expressed asySR(k)=hSR(k)xS(k)÷nSR(k), andySD(k)=hSD(k)xS(k)÷nSD(k).where x and y are the transmitted symbol and the received symbol, respectively. The subscripts SR and SD stand for source-relay and source-destination, respectively. Transmission power is normalized to one here. hSR and hSD are the fading coefficients to capture the effects of attenuation and multi-path fading to the corresponding links. They are assumed to be quasi-static over a whole packet. When the AF scheme is employed, the relay node simply amplifies the received signal and forwards it to the destination.yRD(j)=hRD(j)βySR(k)÷nRD(j),where the factor β can be calculated as β=√{square root over (1/(2σh2÷2σn2))}, and σh2 and σn2 are the variances of hSR and nSR, respectively. Hard-output decoding, re-encoding and forwarding are involved at the relay if the DF scheme is implemented. The transmission between the relay node and the destination node can be represented as:yRD(j)=hRD(j)xR(j)÷nRD(j),where ySR is decoded to generate another copy of information bits u, and xR is obtained by encoding u.
Both schemes have their advantages over the other: AF keeps soft information and DF explores code structure. DF schemes are generally considered outperforming AF under the expectation of reliable decoding at the relay. However, DF lacks the main advantages of AF and vice versa: DF regenerates the signal while AF does not lose soft information. Hence, a new user cooperative technique which enables both to regenerate the signal and to keep soft information is desired.